feat(nextjs): Add deleteSourcemapsAfterUpload option

[lforst]

Will need to wait on: getsentry/sentry-javascript-bundler-plugins#547

---

Adds an easy way to delete sourcemaps immediately after uploading them:

module.exports = withSentryConfig(nextConfig, {
  sourcemaps: {
    deleteSourcemapsAfterUpload: true,
  },
});

Fixes #8600
Fixes #4489